1. Field of the Invention
The present invention relates to a method for processing cereals, seeds or like products by means of a disk mill into such a state that the botanical constituents of such products are separable by means of sifting devices, and the invention also comprises a plant for carrying out the method.
2. Description of the Prior Art
Before the appearance of roller mills, use was generally made of millstones for milling cereals. In a broad sense, a rotatable millstone with a circular working surface may be regarded as a disk, but no one would hardly contend that our old mills with millstones are disk mills.
Even at the time the old mills with millstones were used, efforts were made to improve the milling efficiency in order to reduce the number of passages required for complete milling, by grooving the working surfaces of the millstones. The breakthrough for roller mills meant a considerable improvement in regard of flour quality, yield and production capacity, however at the cost of a very large plant complexity with substantial investment and operating costs.
At regular intervals, suggestions have therefore been made to replace the complicated and expensive roller mills with fast-operating disk mills which in other fields, particularly in the papermaking industry, have gained a reputation of being relatively uncomplicated and having a high production capacity. U.S. patent specification No. 706,259 (Schweitzer) of 1902 is an early example of the idea of using disk mills for milling cereals. Schweitzer suggested the use of annular milling disks with grooves formed in the working surfaces and arranged in groups in a special manner, the primary feature being that the grooves should be defined by sawtooth-shaped ridges with sharp edges and a progressive transition from grooves of a large width and a large depth at the inner periphery of the annular milling disks, where the product to be milled is introduced between the disks, to increasingly finer grooves in a direction toward the outer periphery of the disks. The larger grooves at the inner periphery were meant to facilitate the entrance of the material to be milled, and in the region of these larger grooves, the dehulling of the seeds should start, whereupon the processing should proceed to complete milling in the increasingly finer grooves. Schweitzer also suggested that the grooves should be disposed as tangents to a small circle having the same center as the center of the milling disk in order that the angles of intersection of the grooves in two cooperating milling disks with mutually reversed working surfaces should vary in the circumferential direction of the disks. The concept of milling grain to fine flour in a single step with milling disks of this type did not meet with much success. Shortly afterwards, it was however suggested in U.S. patent specification No. 934,457 (McLaughlin) of 1905 to use a very similar type of milling disks for hulling only. This patent specification does however not give any other indication on the idea of the grooves than that they should be more easy to produce, and it is not explained therein why the proposed groove configuration should be better suited for hulling than milling. One might guess that McLaughlin, being aware of Schweitzer's previous milling disks, knew that they were not suited for milling but could be used for hulling.
Many suggestions of providing milling disks with grooves of different types have thereafter been put forth, however with poor results, at least as regards milling of cereals in a single step for producing high-quality flour and at a low cost. K-E. A. Johnsson states e.g. in SE patent specification No. 419,945 as late as 1981 that there is nothing in a disk mill, at a certain speed of rotation and with a certain milling disk type, to affect the pattern of movement and residence time of the milling material between two coaxial milling disks rotating with respect to each other, and setting out from that conviction, he proposed an entirely novel type of milling disks characterized by arranging in a stationary milling disk a series of small, separately driven milling disks mounted rotatably in pockets in the stationary main disk. The milling result of these new milling disks has not yet been evaluated, but obviously these milling disks are complicated and very sensitive to disturbances.
A feature common to all mills with complicated milling surface configuration is the difficulty of correctly assessing the influence of each separate factor among the complex of factors affecting the milling result. A basic condition for appreciating the milling result of course is that the milled product can be analyzed and that the results of analysis can be related back to the different factors which have been decisive of the milling result. It is also of great importance that the analyses can be effected as quickly as possible and in close proximity of the equipment used in each particular case.
As is well known, the botanical constituents of cereals (seeds and kernels) are the starch body (endosperm minus aleurone layer), the germ and the aleurone and hull layers, said two layers being classified as bran. The hull and aleurone layer fractions and the germ fraction may be used, e.g. as animal fodder or as an additive in a certain amount to the flour as an addition to the fiber content and for increasing the mineral and vitamin content of the flour. The hull and aleurone layers and the starch body of the grain or seed kernels are of different hardness and density, and the very milling process serves to break down the grain or seed kernels to such an extent that the resulting particles can be separated into fractions containing the desired percentages of starch, aleurone and hull fractions. For effective milling and separation for producing a high-quality flour with a satisfactory yield, it has hitherto been necessary to carry out both the milling and the sifting operation in several steps. Certain initial successes in the attempts of replacing the roller mills with disk mills aroused hopes that it would be possible to make the equipment on the mill side of a cereal processing plant less complicated than the machine equipment required for milling in roller mills. However, before the conception of the present invention, these hopes have not been redeemed. This has been confirmed by recent, highly improved methods of analysis.
In SE patent specification No. 419,945 mentioned above, the main reason is assumed to derive from the difficulty of affecting the pattern of movement and residence time of the material milled between the milling disks previously used. The present invention sets out from the assumption that a decisive or strongly contributory reason is that it has been considered necessary for a sufficient breakdown in a single mill passage to let the milling material pass radially between milling disks which comprise a plurality of more or less distinct grooved zones where the grooves are of mutually different depth and width. Milling disks comprising two or more grooved zones necessitate greater radial dimensions than a uniformly grooved single zone. The different grooved zones may however be compared to several milling steps, and it is much more difficult, not to say impossible, by adjusting one milling disk with respect to the other, to correctly adjust one annular grooved zone without affecting adjacent annular grooved zones. An improvement of the breakdown between two opposing annular grooved zones therefore tends to be accompanied by a deterioration of the processing between the next or preceding grooved zone. Under such circumstances, it is very difficult to find out the actual reason or combination of reasons giving the poor result.
A most likely reason might be the difficulty to comply with the requirement for an equivalent breakdown of grains or seeds of varying size in a certain charge milled in successive passes between two or more grooved zones where the groove width and groove depth are gradually decreasing in the direction of movement of the material when being milled. Besides, in the disk mills hitherto known and used it is usually necessary frequently to exchange the milling disks even at minor differences between different charges of material to be milled.
One of the objects of the invention is to provide a method of milling cereals by means of a disk mill which comprises a pair of more or less planar milling disks or, optionally, conical milling surfaces which, like the working surfaces of typical disk mills, cooperate over an essential area as opposed to the tangential cooperation which is typical of roller mills, and to overcome by this method the above-indicated shortcomings and limitations of disk mills and to allow unobjectionable milling during a single passage between two milling surfaces of the type described above and rotating with respect to each other. A further object of the invention is to make it possible, by a relatively simple adjustment of the mill, to process different cereal and seed types within the entire range from dehulling to the production of flour with equally high quality and product yield as in milling by means of conventional roller mills, but at a higher production rate and by the use of a machine equipment which is substantially less complicated and less comprehensive than the equipment which is normal for a roller mill.
Yet another object of the invention is to allow such milling of the kind mentioned above in a single passage through the disk mill that the milled material is discharged from the mill sufficiently disintegrated in respect of its botanical constituents to be readily separable by relatively simple methods of separation into fractions having any desired percentage proportion of the botanical constituents of the seed or cereal fed to the mill.
As already mentioned in the foregoing, the present invention sets out from the experience that previously known, pairwise cooperating milling surfaces of the surface-cooperating type, generally the milling surfaces of typical milling disks, do not yield the desired results, especially at single-step milling, because of unsatisfactory groove configurations and do not permit a correct adjustment of the milling surfaces in relation to each other for ensuring the breakdown and release of the above-described botanical constituents with respect to each other, which is desirable before the subsequent sifting operation. Particularly, the invention has for its object to dispense with the use of a grooved zone with large grooves on the inlet side, and increasingly finer grooved zones towards the outlet or discharge side.
A further object of the invention is to provide a milling plant which comprises a disk mill for carrying out the method and by means of which the processing can be carried out in a simple, adjustable manner for each particular degree of breakdown desired from hulling to fine grinding, such that the subsequent separation into desired fractions or fractional combinations can be effected in a particularly simple and efficient sifting device.
These objects have now been achieved by a method and an apparatus according to the invention which have been given the features recited in the claims.